Endocranial cast of Metaxytherium (Mammalia: Sirenia) from the Miocene of Cerro Gordo, Almería, Spain

Abstract

A natural endocranial cast of Metaxytherium sp. from the Lower Miocene of Cerro Gordo, Almería, Spain, is described. The volume of the cast totals 500 cm3, corresponding to the endocranial volume of the recent Dudong. In the same way as the volume, the shape of the Halitheriinae brain changes very little from the Miocene to the recent age. An exception is constituted by the endocranium of Stellers sea cow, Hydrodamalis gigas, which with 1.650 cm3 exceeds all fossil and recent species. Among the cranial nerves, the nervus opticus and trigeminus have increased in size. There are clear quantitative and qualitative differences between the Miocene and Eocene Halitheriinae. The latter display somewhat smaller, narrower brains and a more modest development of the frontal lobes. For the time being, calculations are available only for the cephalization of the recent species of Sirenia.